The present invention relates to a process for preparing a silicon carbide sintered body having a purity sufficient for its use in equipment for manufacturing semiconductor devices (hereinafter merely referred to as "semiconductor equipment").
Semiconductor equipment is used to carry or handle silicon wafers or similar substrates or shield them from air in the manufacture of semiconductor devices such as integrated circuits.
One type of semiconductor equipment is mainly used during heat treatment steps at high temperatures such as oxidation, CVD, PVD, SOI (silicon on insulator), or thermal diffusion of an impurity as a dopant. Such semiconductor equipment includes wafer carriers (called boats) such as wafer boats, mother boats, and vertical boats as well as tubes and other parts such as process tubes, liner tubes, and forks. Another type of semiconductor equipment is used for securing and positioning and includes hands, vacuum chucks, bell jars, and spacers.
Semiconductor equipment which has conventionally been used is usually made of quartz glass (fused silica) or silicon. Semiconductor equipment made of quartz glass is susceptible to deformation or distortion during heat treatment since quartz glass has a relatively low softening point in the vicinity of 1100.degree. C. Furthermore, when kept at a high temperature for a prolonged period, quartz glass may become devitrified and broken due to phase transition into .alpha.-cristobalite. Therefore, when used for heat treatment, such semiconductor equipment has a quite limited service life under the conditions existing during heat treatment at high temperatures.
Semiconductor equipment made of silicon has problems which should be eliminated from an industrial viewpoint, including low toughness which places many restrictions on fabrication of the equipment.
Silicon carbide (SiC) is chemically stable and resistant to corrosion at high temperatures and its strength and stiffness are much higher than those of quartz glass. In view of these properties, semiconductor equipment made of sintered silicon carbide is sometimes used. However, such semiconductor equipment is rarely used in processing of silicon wafers of high quality, which are sensitive to metallic impurities, since presently available silicon carbide of high purity still contains a considerable amount of metallic impurities. These impurities often evaporate during thermal diffusion treatment, generating gases that cause contamination of wafers.
Accordingly, there is a need for pure silicon carbide powder that is substantially free from metallic impurities, i.e., with a content of 1 ppm or less for each metallic impurity, as a starting material for sintered silicon carbide.
Silicon carbide has two crystal forms, .alpha.-form (hexagonal) which is stable at higher temperatures and .beta.-form (cubic) which is stable at lower temperatures. Of these, .beta.-silicon carbide is more suitable for use in the manufacture of semiconductor equipment since it readily makes a more uniform and pure powder by an industrial process. Known methods for the preparation of silicon carbide powder involve (1) a reaction of SiO.sub.2 with C, (2) a reaction of metallic Si with C, or (3) a vapor phase reaction of a Si compound, e.g., SiCl.sub.4, with a hydrocarbon. Method (1) is used in commercial production of silicon carbide powder since the starting materials are inexpensive and the reaction can be easily controlled.
The most popular process for the preparation of silicon carbide powder using the above-described reaction (1) is the Acheson process. The Acheson process comprises reacting a siliceous material (SiO.sub.2 or its precursor) and a carbonaceous material (C or its precursor) by heating a mixture of these two solid materials in powdery form in a batchwise electric-resistance furnace known as a Acheson-type furnace to produce silicon carbide in lumps.
The Acheson process has a significant drawback that the product is inevitably contaminated with a considerable amount of metallic impurities, which not only come from each of the solid starting materials, which contain an appreciable amount of impurities, but also result- from pulverization which must be performed to finely divide the silicon carbide product in lumps. The Acheson process has additional drawbacks of poor operating efficiency and a deteriorated work environment since it requires the removal of a side wall of the furnace for the recovery of the product in each reaction cycle.
In order to improve the operating efficiency, it has been proposed in Japanese Patent Publication No. 58-18325(1983) and No. 58-34405(1983) that a mixture of the powdery starting materials be shaped by use of a binder such as pitch, thereby making it possible to directly produce a .beta.-silicon carbide powder without a pulverization step. Japanese Patent Application Kokai No. 61-6110(1986) discloses an improved continuous process for the preparation of .beta.-silicon carbide powder which comprises preparing a starting mixture consisting of a solid siliceous material, a solid carbonaceous material, a liquid hydrolyzable silicon compound, and a curable organic compound having polymerizable or cross-linkable functional groups, preheating the starting mixture so as to cure the organic compound and solidify the mixture, and heating to allow the solidified mixture to react in a non-oxidizing atmosphere.
It is also known that a starting mixture is made uniform by using a liquid starting material in order to produce a silicon carbide powder having a uniform particle diameter or shape. For example, it is proposed in Japanese Patent Application Kokai No. 57-8019(1982) to prepare a starting mixture by treating a carbonaceous material with a silicic acid solution and heating the mixture in a non-oxidizing atmosphere. Preferably, the carbonaceous material is also a liquid substance and the mixture is prepared in a liquid state. Unfortunately, a small amount of silica sol is formed in the starting mixture and adversely affects the quality of the product. In order to eliminate this drawback, it is disclosed in Japanese Patent Publication No. 1-42886(1989) to use a mixture comprising a liquid siliceous material, a curable or polymerizable liquid organic compound capable of forming carbon upon heating, and a polymerization or curing catalyst which is compatible with the liquid organic compound to form a homogeneous solution. The mixture is made to react to form a cured body containing Si, O, and C as an SiC precursor, which is then heated in a non-oxidizing atmosphere to give a .beta.-silicon carbide powder.
However, in the above-described prior-art processes for preparation of silicon carbide powder, it is difficult or impossible to produce a silicon carbide powder of high purity having a content of 1 ppm or less for each metallic impurity, a purity level which is acceptable in the manufacture of semiconductor equipment. Therefore, silicon carbide powders obtained in these processes are always contaminated with metallic impurities in considerable amounts, e.g., on the order of 3 ppm or more for one or more impurity metals, and this level of contamination is not acceptable in semiconductor equipment. Although washing is an effective means for removing impurities, it is quite difficult in a commercial process to decrease an impurity level to 1 ppm or less by washing. As a result, a sintered body prepared from a silicon carbide powder made by prior-art methods will not have a desired level of impurity content of 1 ppm or less for each metallic impurity.